lsp: Add support for textDocument/documentSymbol (#27488)

This PR adds support for retrieving the outline of a specific
buffer/document from the LSP.
E.g. for this code (`crates/cli/src/cli.rs`):
```rs
use collections::HashMap;
pub use ipc_channel::ipc;
use serde::{Deserialize, Serialize};

#[derive(Serialize, Deserialize)]
pub struct IpcHandshake {
    pub requests: ipc::IpcSender<CliRequest>,
    pub responses: ipc::IpcReceiver<CliResponse>,
}

#[derive(Debug, Serialize, Deserialize)]
pub enum CliRequest {
    Open {
        paths: Vec<String>,
        urls: Vec<String>,
        wait: bool,
        open_new_workspace: Option<bool>,
        env: Option<HashMap<String, String>>,
    },
}

#[derive(Debug, Serialize, Deserialize)]
pub enum CliResponse {
    Ping,
    Stdout { message: String },
    Stderr { message: String },
    Exit { status: i32 },
}

/// When Zed started not as an *.app but as a binary (e.g. local development),
/// there's a possibility to tell it to behave "regularly".
pub const FORCE_CLI_MODE_ENV_VAR_NAME: &str = "ZED_FORCE_CLI_MODE";
```

Rust-analyzer responds with:
```
Symbol: 'IpcHandshake' - Struct - (4:0-8:1) (5:11-5:23)
  Symbol: 'requests' - Field - (6:4-6:44) (6:8-6:16)
  Symbol: 'responses' - Field - (7:4-7:48) (7:8-7:17)
Symbol: 'CliRequest' - Enum - (10:0-19:1) (11:9-11:19)
  Symbol: 'Open' - EnumMember - (12:4-18:5) (12:4-12:8)
    Symbol: 'paths' - Field - (13:8-13:26) (13:8-13:13)
    Symbol: 'urls' - Field - (14:8-14:25) (14:8-14:12)
    Symbol: 'wait' - Field - (15:8-15:18) (15:8-15:12)
    Symbol: 'open_new_workspace' - Field - (16:8-16:40) (16:8-16:26)
    Symbol: 'env' - Field - (17:8-17:44) (17:8-17:11)
Symbol: 'CliResponse' - Enum - (21:0-27:1) (22:9-22:20)
  Symbol: 'Ping' - EnumMember - (23:4-23:8) (23:4-23:8)
  Symbol: 'Stdout' - EnumMember - (24:4-24:30) (24:4-24:10)
    Symbol: 'message' - Field - (24:13-24:28) (24:13-24:20)
  Symbol: 'Stderr' - EnumMember - (25:4-25:30) (25:4-25:10)
    Symbol: 'message' - Field - (25:13-25:28) (25:13-25:20)
  Symbol: 'Exit' - EnumMember - (26:4-26:24) (26:4-26:8)
    Symbol: 'status' - Field - (26:11-26:22) (26:11-26:17)
Symbol: 'FORCE_CLI_MODE_ENV_VAR_NAME' - Constant - (29:0-31:67) (31:10-31:37)
```

We'll use this to reference specific symbols in assistant2

Release Notes:

- N/A

crates/lsp/src/lsp.rs

@@ -774,6 +774,10 @@ impl LanguageServer {
                     code_lens: Some(CodeLensClientCapabilities {
                         dynamic_registration: Some(false),
                     }),
+                    document_symbol: Some(DocumentSymbolClientCapabilities {
+                        hierarchical_document_symbol_support: Some(true),
+                        ..DocumentSymbolClientCapabilities::default()
+                    }),
                     ..TextDocumentClientCapabilities::default()
                 }),
                 experimental: Some(json!({